Do insects learn

A common notion about insects is that they are wholly instinct driven in their behaviour like a programmed tank (they are referred to as tanks because of the fairly thick and hard armour of chitin and protein covering their body). That was the basic notion, which we find in the writings of Jean-Henri Fabre and Henri Bergson. In other words, there is no place for learning in insects. Learning is a necessary factor in expression and manifestation of intelligence. Intelligence is the property of using elements of the past learning and experience to solve new problems. It is a general presumption that insects do not have intelligence and that they are fully instinct guided.

But of late such facts have been emerging as suggest that insects, too, have some capacity of learning. O'Donnell and his colleagues have published their observations (O'Donnell et al, 2004), which are of interest in this context. They have studied the brain of the social wasp Polybia aequatorialis, which lives in fairly large colonies, a colony including 2000 or more workers. The workers show well marked changes in behaviour as they grow older. Young workers work in the interior of the nest, mainly taking care of developing individuals. After growing older, they start working on the nest exterior, adding more material to the nest cover and defending the colony. When they are still older, they start flying out for collection of food and building material.

With this change of work and behaviour there are marked changes in the brain of the worker wasps. O'Donnell and his team have found that mushroom bodies of their brain progressively increase in size, and they show maximum increase, when the workers start working on the nest exterior. With this change of tasks, their life becomes more challenging. O'Donnell says, "What is happening is that the complexity of the tasks the insect engage in is increasing. They are going from living in a very constrained spatial area with dim light to working outside the nest where is a complex sensory environment with higher light levels. Finally, they have to leave the nest to forage for materials and then to locate their way home to the nest".

It is interesting to note that similar observations have been made in man. In the January 2004 issue of Nature, Dr. Arne May and his coworkers (Anonymous, 2004) studied the brain of persons who were getting training in juggling. The investigators used the technique of magnetic resonance imaging (MRI). They noted that, as the jugglers learnt their new lessons, certain areas in the cerebral cortex showed enlargement. When the subjects gave up juggling completely, the areas, which had shown enlargement, started shrinking. These observations were similar to those which had been made some years earlier on apprentice taxi drivers, who had been asked to move on their bicycles through different parts of London, holding the city map, to become familiar with streets and lanes. These trainees also showed enlargement of the cortex in the hippocampus part of the brain as they went ahead with the training.

The cortex of human brain is a layer of the grey matter, which is made up of cell bodies of neurons or nerve cells, while in deeper parts is the white matter, which consists of nerve processes of neurons. It is not clear whether enlargement of certain parts of the brain during training had been due to formation of new neurons or enlargement or hypertrophy of existing neurons, because MRI is not able to go down to the cellular level. But one thing may be inferred from the above observations, namely that the cortex enlargement is associated with learning and formation of new reflexes. Thus, it may be safely stated that enlargement of certain parts of the brain in the social wasps, as they take to more demanding jobs, is due to learning. There are some other observations, too, on biology and behaviour of insects, which suggest some learning and intelligence, while most of their activities are governed by instincts (see the previous chapter on "Instinct and intelligence in insects").

Several books have been published recently on learning in insects (Abram-son et al, 1990; Papaj and Lewis, 1993 and others). As Papaj (2003) mentions, a butterfly learns to search for the shape of its preferred host plant's leaves and for the colour of preferred nectar sources. A grasshopper avoids feeding on a plant associated with recent digestive problems. The same can be said about parasitoid wasps, flies and bees. In Tribolium beetles and Drosophila flies memory formed in the larval stage persists during metamorphosis. Most of the experiments have been done with bees and wasps (Bitterman, 1996; Menzel, 2001).